Streptokinase immobilized covalently onto nylon and implanted subcutaneously into a rabbit has been shown to retain its activity in increasing the blood clotting time for more than 150 days. No adverse side effects in the animal were noted during this period and no evidence indicating an immune response was found. This suggests that immobilized streptokinase may be useful in the prevention of thromboembolic disorders. During this year we intend to continue our in vivo studies with rabbits to demonstrate the fibrinolytic activity of the immobilized streptokinase in the prevention of the formation of induced clots as well as in dissolving existing clots. At the same time we will determine the effects on various blood-clotting factors including fibrinogen, fibrin degradation products, plasminogen and plasmin. In addition, we will determine the extent of any immune response to the immobilized streptokinase using more sensitive techniques. We further intend to immobilize streptokinase on the inside of vessel prostheses, implant these prostheses in dogs and evaluate to what extent immobilized streptokinase can be used to inhibit the development of the intimal proliferation and maintain the size of the lumen. Our results will aid in the evaluation of the usefulness of immobilized streptokinase in the prevention and treatment of thromboembolic disorders.